Investigation of the stereodynamics of tris-(α-diimine)–transition metal complexes by enantioselective dynamic MEKC

Abstract

Enantiomerization of octahedral tris(α-diimine)–transition metal complexes was investigated by enantioselective dynamic MEKC. Varying both the transition metal ion (Fe²⁺, Fe³⁺, and Ni²⁺) and the bidentate diimine ligand (1,10-phenanthroline and 2,2′-bipyridyl), the enantiomer separations were performed either in a 100 mM sodium tetraborate buffer (pH 9.3) or in a 100 mM sodium tetraborate/sodium dihydrogenphosphate buffer (pH 8.0) both containing sodium cholate as chiral surfactant. The unified equation of dynamic chromatography was employed to determine apparent reaction rate constants from the electropherograms showing distinct plateau formation. Apparent activation parameters ΔH^‡ and ΔS^‡ were calculated from temperature-dependent measurements between 10.0 and 35.0°C in 2.5 K steps. It was found that the nature of the central metal ion and the ligand strongly influence the enantiomerization barrier. Surprisingly, complexes containing the 2,2′-bipyridyl ligand show highly negative activation entropies between −103 and −116 J (K mol)⁻¹ while the activation entropy of tris(1,10-phenanthroline) complexes is positive indicating a different mechanism of interconversion. Furthermore, it was found that the Ni²⁺ complexes are stereostable under the conditions investigated here making them a lucent target as enantioselective catalysts.